The transforming growth factor (TGF)-Betas are potent growth inhibitors of normal mammary epithelial cells. Constitutive overexpression of TGF- Beta1 in mammary epithelium of transgenic mice prevents carcinogen- and oncogene-induced mammary tumors. In established breast cancer cell systems, however, tumor cell TGF-Betas foster host/tumor interactions which indirectly support the viability and/or progression of carcinomas. In addition, high levels of TGF-Betas in human breast tumors have been associated with poor patient outcome and a more malignant breast cancer phenotype. Thus, we hypothesize that the net effect of TGF-Betas changes during stochastic mammary epithelial carcinogenesis. This net effect depends on the balance between autocrine growth inhibition, prevalent in normal breast cells, and a more permissive yet indirect role in transformed cells, associated or not with loss of endogenous TGF-Beta receptors. This proposal will therefore attempt to define the timing and mechanisms by which TGF-Betas modulate mammary carcinogenesis and, later, the progression of established tumors. In the first Aim, we will develop a transgenic mouse model in which levels of mammary epithelial TGF-Beta1 expression can be temporally controlled. In the second Aim, we will use this model to examine the effect of conditional expression of breast epithelial TGF-Beta1 on early and late stages of mammary transformation and tumor progression induced by carcinogens. In the third Aim, by using temporally-regulated active TGF-Beta1 or TGF- Beta2 as well as antisense TGF-Beta 1 or TGF-Beta2 in transfected human breast cancer cell lines in nude mice, we will test the effect of overexpression or elimination of TGF-Betas. The fourth Aim will dissect the paracrine interactions between tumor cells and host immune, endothelial, and stromal cells, which we propose may account for the net effect of TGF-Betas on mammary tumorigenesis. Finally, the fifth Aim will examine how tumor cell TGF-Betas enhance resistance of breast tumor cells to chemotherapeutic drugs perhaps through interactions with stromal cells and the extracellular matrix. The results from these studies will lead us to re-examine the dynamic role of TGF-Betas in progressive mammary transformation and, on the basis, the conception of optimal chemoprevention strategies. In addition, a marked alteration in the natural history and drug resistance of experimental tumors by elimination of tumor cell TGF-Betas would suggest these molecules as rational and testable targets for novel antitumor interventions in patients with breast cancer.